Ultraviolet absorbance spectroscopy has been widely used to quantitatively detect levels of aromatic compounds in solution. Individual aromatic compounds have characteristic spectra which differ with ring size and substitution of the aromatic. The ultraviolet absorbance of the sample is proportional to the product of the molar concentration and the pathlength. The constant of proportionality is known as the extinction coefficient. The extinction coefficient varies widely among compounds. Therefore, two approaches to quantification of mixtures of aromatics have been taken. In one, the spectra of the individual compounds present are "deconvoluted" by mathematical analysis and simulation of the individual overlapping absorbance bonds. This method is limited to a small number (typically less than 10) of compounds in a mixture. If the nature of compounds is unknown, there is much uncertainty as to whether accurate levels are obtained. In the other approach, the extinction coefficients are replaced with "response factors" which characterize the average value of extinction coefficient which is expected to apply. When the relative proportions of compounds comprising the mixture changes, the "response factors" must also be changed.
Reference is made to Klevens and Platt, J. Chem. Phys. 17:470 (1949). Similarities are reported in the total oscillator strength for electronic transitions of cata-condensed aromatics. In that report, the similarities were used to support a theory for the quantum mechanical basis of electronic transitions in known structures. No realization was made that this method could be applied to measure the level of aromatic functionality in mixtures of unknown structures.
Furthermore, cata-condensed aromatics constitute only a minor fraction of the various aromatics in a hydrocarbon feedstock such as petroleum. Other aromatics, including peri-condensed aromatics, alkyl aromatics, naphtheno-aromatics and thiophenic aromatics which behave spectroscopically differently from cata-condensed aromatics, are also generally present in a hydrocarbon feedstock.
Also, the article does not relate to HPLC analysis, nor does it recognize or suggest that a UV detector operating in a specific wavelength range can be used in HPLC to derive an integrated oscillator strength output which quantifies the aromatic carbon in petroleum and shale oil feedstocks.
Chromatography is a well-documented and widely used laboratory technique for separating and identifying the components of a fluid mixture, e.g., a solution, and relies on the different relative affinities of the components between a stationary phase and a mobile phase which contacts the stationary phase.
In a typical example of chromatography, the stationary phase is a suitable particulate solid material which is substantially uniformly packed into a tube so as to form a column of the stationary phase material. The mobile phase may be the fluid under investigation, or more commonly, a solution of the fluid under investigation. The solvent used in the solution is usually first passed through the column of solid stationary phase and thereafter a small sample comprising a solution of the fluid under investigation is passed through the column, followed by solvent alone. The components of the fluid will have different affinities for the stationary phase and will therefore be retained at different regions along the length of the column for different times. For some components, the affinity will be so slight that virtually no retention is evident while for others, the affinity might be so great that the components are not recovered from the column even after considerable periods of time have elapsed since they were introduced into the column and subjected to the potential eluding properties of the solvent.
Petroanalysis 81, Chapter 9, discloses that a hydrocarbon mixture combined with a solvent results in an eluate being recovered from the exit end of a chromatographic column which comprises the following types of molecular species, in order, namely: saturates (e.g., paraffins and naphthenes), olefins and aromatics. The remaining molecular species, generally polar compounds, have a relatively high affinity for the solid chromatographic material and can only be recovered in a reasonable time and reasonably completely by interrupting the flow of the solvent and substituting a different solvent having a relatively high affinity for, e.g., heteroaromatic compounds. The different solvent is passed through the column in a direction opposite to that of the first solvent (back-flushing) so that after a reasonable time interval, polar compounds (resins) are present in the back-flush eluate. The change in solvent from the first solvent, pentane, to the second solvent, methyl t-butylether, necessitates the use of two different eluent detectors, i.e., one using refractive index and the other using ultra-violet absorbance at 300 nm.
In Journal of Liquid Chromatography, 3(2), 229-242 (1980), a hydrocarbon mixture containing asphaltenes is subjected to chromatographic analysis only after mixing with hexane to precipitate asphaltenes which are separated by filtration and then determined gravimetrically. The hexane solution of the remaining hydrocarbons is then passed through a column of particles of u-Bondapak-NH.sub.2 where it separates into an eluent comprising, initially, saturates and then aromatics, as determined by the refractive index of the eluent. Resin which is retained on the column is backflushed off the column and determined by difference. The separation quality of the column is maintained by flushing it with a solution of 1/1 methylene chloride/acetone after every 20 samples and then regenerating with methylene chloride and hexane for repeatable retention times. Changes in the refractive index of the eluents, indicative of the presence of respective chemical species, are monitored and correlated with absolute amounts of the chemical species by means of a Hewlett-Packard 3354B computer using the so-called "Zero" type method.
In Journal of Chromatography, 206 (1981) 289-300, Bollet et al., a rapid high-performance liquid chromatography technique for separating heavy petroleum products into saturated, aromatic and polar compounds is described. A column containing a stationary phase of silica bonded NH.sub.2 ("Lichrosorb NH.sub.2 ") is used. Two chromatographic analyses are needed in order to determine the composition of a sample. In the first analysis, saturated compounds are separated from aromatic and polar compounds, using hexane or cyclohexane as the mobile phase. In the second analysis, saturated and aromatic compounds are separated from polar compounds using 85 vol% cyclohexane, 15 vol% chloroform as the mobile phase. The eluents are monitored by differential refractometry for saturated, aromatic and polar compounds, and by ultraviolet photometry for polar compounds. The proportions of saturated and polar compounds are said to be determinable by these monitoring techniques and the proportion of aromatic compounds found by difference. However, the method described, in common with all other reports of high performance liquid chromatography for analysis of samples of heavy hydrocarbon oil mixtures, is limited by the lack of a means and method for quantitative and feedstock-independent detection and monitoring. Thus, for both refractive index (RI) and ultraviolet (UV) detectors, "response factors" must be derived by separating samples of the feedstock on a larger scale, known in the art as "semi-preparative liquid chromatography" and then gravimetrically weighing the recovered analyte (after removal of the solvent(s) added to the sample for the purpose of the chromatographic separation). Response factors are dependent on the nature of the feedstock and its boiling range, and it is therefore essential to perform the relatively large-scale separation to obtain accurate results with the HPLC analysis. Thus, the potential benefits of speed and increased resolution which should be possible with HPLC have not heretofore been fully realized in practice.